Cold radiators in loft

[milleniumaire]

Hi, I've have a problem with my CH system for around 18 months now and it's time to get it sorted. Here's a diagram showing our current heating system:

The house is 21 years old and the loft extension was performed around 15 years ago. As you can see they simply extended the existing 15mm pipes up into the loft and then split them. The Vaillant equipment is 8 years old and still going well. We had the UFH installed 5 years ago as part of a kitchen extension.

Only one of the radiators in the loft gets a little warm, never hot and the other radiator and towel rail are always cold, so there's clearly a blockage or two. I have accessed the 15mm flow and return pipes to the loft at the point they are connected under the bathroom floor and the flow pipe gets very hot, the return pipe is barely warm. I have tried closing all valves to force the water into the loft pipes but it makes no difference.

I have been considering replacing the existing 15mm pipes into the loft with 22m pipes and connect them up earlier as is shown in the following diagram:

Obviously I'll have to resolve the existing blockages in the loft pipework.

My diagram may not be accurate for the Ground floor radiators. It currently shows the flow going into the final toilet radiator and then back out into the return. Is this how you would expect the pipes to be connected at the end of a run or would the flow become the return? The 1st Floor to loft connections shown in the first diagram are exactly as they are plumbed. So, if all the loft radiators were turned off, there's no way for the flow to return to the boiler (I think!). In fact, in my diagram the only way for the flow to become the return is through a radiator. Not sure how accurate this is.

A couple of concerns:
1. Will the boiler pump be able to handle the additional 22mm pipes running into the loft?
2. Should I consider re-routing (new) pipes around the loft as the "star" connection currently used looks to be a limitation to my untrained eye.

Thanks in advance for your thoughts.

 

[centralheatking]

Great diagrams well done. 15mm pipe is as you suspect the limitation. Putting in 22mm
will put less strain on the pump. Yes pipe up the loft with conventional flow and return.
centralheatking

 

[Gasmk1]

Have you actually bled the rads could be as simple as air or an air lock

 

[Stigster]

No matter what you shut off, every rad has a flow and return path to boiler.

Undersized pipework is an issue but you'd still expect them to all heat up fairly evenly if not as hot as they should. It does sound like a possible airlock as Gasmk1 says.
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Another thought, I know you have tried closing all the other rads down to force out any airlocks. However when that fails to work I would temporarily increase the system pressure to 2 or 2.5 bar which will compress down any slugs of air in the system and give the pump a better chance at moving air to a radiator where it can be bled off.

Return to normal operating pressure after trying that of course.

 

[milleniumaire]

Have you actually bled the rads could be as simple as air or an air lock

No matter what you shut off, every rad has a flow and return path to boiler.

Undersized pipework is an issue but you'd still expect them to all heat up fairly evenly if not as hot as they should. It does sound like a possible airlock as Gasmk1 says.

Hi guys, thanks for responding.

When I first encountered the issue I assumed it was the TRV's that were sticking on the two radiators and so replaced them. While doing this I removed both radiators and gave them a good flush through outside. There was a fair amount of black gunge in them, despite having a magnacelan on the system (oops, forgot to include that in my diagrams ).

Unfortunately, this didn't make any difference. My suspicion is the water flow in these radiators is so poor due to the 15mm pipes connected to the very end of the pipe run and the "star" method of connecting them up, that gunge has collected in the pipes feeding the radiators.

If it was simply air, couldn't that be flushed through by closing off every other radiator to increase the flow/pressure? I have tried this a couple of times.

Great diagrams well done. 15mm pipe is as you suspect the limitation. Putting in 22mm
will put less strain on the pump. Yes pipe up the loft with conventional flow and return.
centralheatking

Thanks for posting. It's strange as I imagined a larger 22mm pipe would put more strain on the pump as it has a larger volume of water to move through the pipe, especially as this new run of 22mm has to go from one end of the house to the other, then up a floor.

 

[Stigster]

If it was simply air, couldn't that be flushed through by closing off every other radiator to increase the flow/pressure? I have tried this a couple of times.

Sometimes the slug of air is too long and compressible to be moved by the pump even under ideal circumstances with the other rads closed. That's why I suggested trying with the pressure increased. It compresses the trapped bubble giving the pump a better chance to move it to a rad.

I only suggest it because it sometimes works and is easy enough to do. Always try the simpler things first, which I appreciate you have been doing but that's one more to try.

 

[milleniumaire]

Another thought, I know you have tried closing all the other rads down to force out any airlocks. However when that fails to work I would temporarily increase the system pressure to 2 or 2.5 bar which will compress down any slugs of air in the system and give the pump a better chance at moving air to a radiator where it can be bled off.

Return to normal operating pressure after trying that of course.

Sorry Stigster I didn't see this original post. I will definitely try this. I assume to increase the system pressure it's simply a case of letting more water into the system and monitoring the pressure gauge. While the boiler is running hot, the pressure is at 1.8 bar, so I guess I could increase this to 2.5 bar and then afterwards let water out through the magnaclean connections.

 

[centralheatking]

Sometimes the slug of air is too long and compressible to be moved by the pump even under ideal circumstances with the other rads closed. That's why I suggested trying with the pressure increased. It compresses the trapped bubble giving the pump a better chance to move it to a rad.

I only suggest it because it sometimes works and is easy enough to do. Always try the simpler things first, which I appreciate you have been doing but that's one more to try.

You could shoot the system up with mains pressure ...find a rad with a good type mt cock
connect hose with a jubilee clip and put the other end on to outside tap. close off boiler
flow and return then shoot it up with mains pressure water, start at the bottom and bleed every rad going upwards and along so the last rad is highest and furtherest away.
In case anybody wants to do this with open vented it works but be carefull not to overfill
the F & E tank. blip the pump on and off ...its a bit like heart surgery for heating systems.
then undo the lot and maybe dump all the water and do again..its not rocket science
centralheatking